1. Field of the Invention
The present invention relates to a toner which is used to develop an electrostatic latent image in an image forming apparatus employing an electrophotographic process or electrostatic printing process, as well as to a method of manufacturing the toner, a two-component developer, a developing device, and an image forming apparatus.
2. Description of the Related Art
A toner is used to visualize a latent image in various image forming processes, and one known example thereof is an electrophotographic image forming process.
In an electrophotographic image forming apparatus, an image is formed by way of the steps of charging, exposure, developing, transferring, and fixing. First of all, in the charging step, a surface of a photoreceptor is uniformly charged. The photoreceptor serves as an image bearing member on which an electrostatic latent image is to be formed. In the exposure step, the charged surface of the photoreceptor is irradiated with light corresponding to image information so that an electrostatic latent image is formed on the surface of the photoreceptor. In the developing step, a toner is attached optionally to the electrostatic latent image formed as above so that an image visualized with toners (toner image) is formed on the surface of the photoreceptor. And in the transferring step, the toner image is transferred onto a transfer sheet by electrical attraction. In the final fixing step, the toner image transferred on the transfer sheet has its toner molten by heat so that the toner image is fixed to the transfer sheet.
As a color imaging technique has been rapidly advanced in the electrophotographic process, a full-color image forming apparatus has been developed and now available in the market. The market scale of the full-color image forming apparatus is expanding along with prevailing black-and-white image forming apparatus. In reproducing colors in the full-color image forming apparatus, it is common to use three subtractive primary colors; yellow (Y), magenta (M), and cyan (C), or four colors composed of the above three colors and black (K). The colors are reproduced in the following procedures. Among the image forming steps, the steps of charging, exposure, developing, and transferring, are repeated for respective colors of C, M, Y, and K so that toner images made of plural colors overlap one another on the transfer sheet, thus forming a full-color image. And in the final fixing step, the above overlapping toner images are fixed to the transfer sheet with the toners being molten thereon. In the procedure just described, the molten toners constituting the overlapping toner images are mixed with each other, with the result that the colors are reproduced based on the subtractive principles.
In the full-color electrophotographic process as described above, the development is conducted more than once and in the fixing step, a plurality of toner images different in color need to be overlaid with one another on the same support, which fact grows importance of expected chargeability, fluidity, and fixing property of the toners in respective colors.
That is to say, in order to maintain stable and favorable color reproducibility for full-color images, predetermined amounts of the toners need to be firstly attached to surfaces of photoreceptors in the developing step, and all of the toners need to be then transferred onto a transfer sheet. The amounts of the toners in the developing step and the transferring step are greatly influenced by the charging properties of the toners such as a charge rising property, and environmental stability and temporal stability of charge amounts in the toner.
For that reason, the toner contains a charge control agent with binder resin, colorant, etc., to improve and adjust the charge amount.
It is known that it is important to uniformly and finely disperse the charge control agent into toner particles or in surfaces of toner particles in order to provide the toner with excellent charging properties.
The toner containing the charge control agent in the toner particles used to have a problem that the charge control agent partially aggregates in the toner or is made not fine enough and exist in form of coarse particles even when uniformly dispersed, thereby causing troubles such as a failure to obtain a sufficient amount of initial charges, a defective charge rising property, or serious charge decay.
Various efforts have therefore been made to uniformly and finely disperse the charge control agent into the toner, but it is difficult to uniformly and finely disperse the charge control agent into the toner and moreover, it is difficult to have the charge control agent optionally exist in the surfaces of toner particles which are important for contact electrification with a charging member or a carrier.
To overcome such difficulties, there has been proposed a method of immobilizing the charge control agent in the surfaces of the toner particles. For example, in Japanese Unexamined Patent Publication JP-A 1-185649 (1989), a mechanical impact is applied and thereby synthetic resin containing a charge control agent is attached to surfaces of toner particles containing colorant and synthetic resin.
In Japanese Unexamined Patent Publication JP-A 2004-109406, fine resin particles are dipped into a solution having a charge control agent dissolved therein, and then dried to deposit the charge control agent, and the fine resin particles are immobilized in toner particles by a mechanical impact.
In Japanese Unexamined Patent Publication JP-A 4-182662 (1992), fine resin particles and a charge control agent are attached to toner particles by a mechanical impact, and to the toner particles thus obtained, a solution is sprayed and then dried inside a hot-air drier.
In each of the toners disclosed in JP-A 1-185649 (1989), JP-A 2004-109406, and JP-A 4-182662 (1992), the charge control agent and the fine resin particles are attached, bonded, or melt-bonded to the toner particles, allowing for enhancement in the chargeability of the toner.
The toners disclosed in JP-A 1-185649 (1989), JP-A 2004-109406, and JP-A 4-182662 (1992), however, have the following problems to be solved.
In the toner disclosed in JP-A 1-185649 (1989), the mechanical impact generated by a commonly-used mixer, e.g., a hybridizer, is used to let the fine resin particles and the charge control agent be buried in and bonded to the toner particles. Since these fine resin particles and charge control agent are immobilized not firmly in the toner particles, the fine resin particles and the charge control agent may fall off the toner particles during the long-term use, for example, by agitation inside a developing container, thus decreasing life stability of charges.
Also in the toner disclosed in JP-A 2004-109406, the mechanical impact is applied and thereby the fine resin particles having the charge control agent attached thereto are buried in and bonded to the toner particles. Since these fine resin particles and charge control agent are immobilized not firmly in the toner particles, the fine resin particles and the charge control agent may undesirably fall off the toner particles during the long-term use, for example, by agitation inside a developing container.
In JP-A 4-182662 (1992), the fine resin particles are attached to the toner by the mechanical impact and then, the solution is sprayed to the toner thus obtained, and therefore the fine resin particles and the charge control agent are immobilized firmly in the toner particles as compared to the toners disclosed in JP-A 1-185649 (1989) and JP-A 2004-109406. However, no mechanical impact is applied in the step of spraying the solution, and a coating layer containing the fine resin particles and the charge control agent is not melt-bonded and fixed to the toner particles and therefore, the fine resin particles and the charge control agent may undesirably fall off the toner particles during the long-term use, for example, by agitation inside a developing container.